1. Field of the Invention
The present invention relates to a lattice filter using a resonator consisting of a crystal vibrator or an elastic surface wave vibrator or the like.
2. Description of the Related Art
FIG. 4 shows structure of a prior art lattice filter. In FIG. 4, between a first input end 11 and a first output end 12, and between a second input end 13 and a second output end 14, there are provided first resonators 15, 15, each having the same characteristic respectively. Also, between the first input end 11 and the second output end 14, and between the second input end 13 and the first output end 12, there are provided second resonators 16, 16, each having the same characteristic respectively.
In this case, the first resonator 15 has a series resonance frequency f1s and a parallel resonance frequency f1p higher than the series resonance frequency f1S, and the second resonator 16 has also a series resonance frequency f2s and a parallel resonance frequency f2p higher than the series resonance frequency f2S. Also, the parallel resonance frequency of one or the other of the first and second resonator, for example f2p of the second resonator 16, is substantially equal to the series resonance frequency f1s of the first resonator 15, which is the other resonator. Accordingly, reactance curves of these resonators 15, 16 become as shown in FIG. 5.
Further, in these two types of resonators 15, 16, as shown in FIG. 5, their reactance curves have such relationship that they cross each other in a region of lower frequencies than the series resonance frequency f2s of the second resonator 16, and that they cross each other in a region of higher frequencies than the parallel resonance frequency f1p of the first resonator 16. Such crossing relationship can be obtained by setting physical dimensions within the respective resonators.
As a result, a transmission characteristic when a signal is inputted between the first input end 11 and the second input end 13 becomes a characteristic of a band pass filter which has between the series resonance frequency f2s of the second resonator 16 and the parallel resonance frequency f1p of the first resonator 16 as the passing band as shown in FIG. 6.
Also, at frequencies fL and fH at which each other""s reactance curves cross, phases at two output ends 12 and 14 become the same and therefore, no signal is generated between these two output ends 12 and 14, but a characteristic which attenuates as shown in FIG. 6 is obtained.
Therefore, in order to widen the passing band, a difference (xcex941=f1pxe2x88x92f1s, xcex942=f2pxe2x88x92f2s) between the series resonance frequency and the parallel resonance frequency of each resonator 15, 16 is made larger, or the parallel resonance frequency f2p of the second resonator 16 is made lower than the series resonance frequency f1s of the first resonator 15, whereby the series resonance frequency f1s of the first resonator 15 can be spaced apart from the parallel resonance frequency f2p of the second resonator 16.
However, the difference between the series resonance frequency and the parallel resonance frequency in each resonator cannot be made much large because of constraints in the physical dimensions, but also for the similar reason, it becomes impossible to freely set a frequency at which the reactance curves cross each other as well.
Also, there is the problem that when the series resonance frequency of the first resonator is spaced apart from the parallel resonance frequency of the second resonator that is higher than the series resonance frequency, a ripple occurs in the transmission characteristic of the passing band.
Thus, in a lattice filter according to the present invention, even when the difference between the series resonance frequency and the parallel resonance frequency of each resonator cannot be made large, it is an object to make it possible to widen the passing band without causing any ripple in the passing band.
As means for solving the above-described problem, there is provided a lattice filter comprising: two first resonators which are substantially equal to each other in the first series resonance frequency and are substantially equal to each other in the first parallel resonance frequency, that is higher than the first series resonance frequency; two second resonators which are substantially equal to each other in the second series resonance frequency and are substantially equal to each other in the second parallel resonance frequency, that is higher than the second series resonance frequency; two first inductance elements which are substantially equal to each other in the inductance; and two second inductance elements which are substantially equal to each other in the inductance, wherein one of each of the first resonator and the first inductance element is connected to each other in series, and is inserted between the first input end and the first output end while the other of each of the first resonator and the first inductance element is connected to each other in series, and is inserted between the second input end and the second output end; and one of each of the second resonator and the second inductance element is connected to each other in series and is inserted between the first input end and the second output end while the other of each of the second resonator and the second inductance element is connected to each other in series and is inserted between the second input end and the first output end, whereby the parallel resonance frequency of the second resonator is caused to substantially coincide with the series resonance frequency based on the first resonator and the first inductance.
Also, a reactance curve based on the first resonator and the first inductance element and a reactance curve based on the second resonator and the second inductance element are caused to cross each other in a region of lower frequencies than a series resonance frequency based on the second resonator and the second inductance element.
Also, the reactance curve based on the first resonator and the first inductance element and the reactance curve based on the second resonator and the second inductance element are caused to cross each other in a region of higher frequencies than the parallel resonance frequency of the first resonator.